The objectives of this research program are to study the epidemiology, natural history and treatment of radiation-induced thyroid cancer by the screening and follow-up of a cohort of 5266 subjects who received therapeutic x-ray exposures to the head and neck region during childhood. The prevalence, risk factors and continuing incidence of thyroid cancer will be established by a long-term follow-up of all traced subjects. 1618 (31%) subjects have already been examined and an additional 1370 (26%) subjects have been contacted by mail and are also available for follow-up study. A prevalence of thyroid nodules of 25% was found in patients examined at Michael Reese Hospital. Of 478 subjects undergoing surgery, thyroid cancer was found in 167 (35%). An additional 199 subjects have not had any specific surgical treatment (about 70 of these may be expected to have thyroid cancer) providing an opportunity to study the natural history of radiation induced tumors. Follow-up of the surgically treated subjects will allow evaluation of the morbidity and mortality of thyroid surgery. Comparison of the results in these two groups will provide a guide to future management of radiation-induced thyroid tumors and their relationship to occult thyroid cancer of no apparent clinical significance which has been observed at autopsy in up to 6% of unirradiated subjects dying from unrelated causes. The findings in this study will have a bearing on the management of the many thousands (or perhaps a million) of people who received therapeutic irradiation for benign conditions in childhood many years ago and on children now being treated for malignancies in the head, neck and chest area who receive similar radiation doses to the thyroid gland.